The cellular, biochemical and architectural organization of the hematopoietic microenvironment will be studied using several experimental models. Trilaminar cellulose ester membranes (CEMs) enriched with spleen homogenate or bone homogenate to support trilineal hematopoiesis will be compared to unenriched trilaminar CEMs which are predominantly granulopoietic. These several types of hematopoietic microenvironments will be studied by electron microscopy, mucopolysaccharide chemical makeup and light microscopy. Sinusoidal microcirculation, fibroblasts, fat cells and reticular cells will be identified by electron microscopy and their relationship to granulopoiesis, erythropoiesis and megakaryocytopoiesis studied. The effect of hemopoietic stress upon this system will be studied in splenectomized, 89 Strontium treated mice who would otherwise die of hematopoietic failure. We will also investigate the ability of the modified CEMs to prevent hematopoietic failure. Cloned bone marrow fibroblasts (plaque-forming cells) and osteoblasts will be cultured in vitro and placed on CEMs for implantation into peritoneal cavities of CBA/CA or CBA/T6T6 mice with chromosome markers. Marrow developing on such CEMs could then be ascribed to the cloned stromal cells or to migrating CFUs.